The nest Protect 05A is part of the CO & smoke alarm
test program at Consumer Reports. In our lab tests, carbon monoxide alarm
models like the Protect 05A are rated on multiple criteria, such as those listed below.

High CO levels:

High CO levels rates a model's response to CO of 300 parts per million.

Low CO levels:

Low CO levels rates a model's response to CO of 150 ppm.

Similar Models

The following models have the same ratings as nest Protect 05A.
Features and specs may vary

Had 4 of them. One kept saying it had a bad sensor. Reset it and it was ok for another month. Every month another one would spout off there is smoke when there wasn't any. You need an IOS or Android device to set them up and does not support Windows Phone. Luckily I am able to send them back and get a refund since the product had a recall because of the 'hand wave' to silence feature.